This invention pertains to hollow, composite-material, rebar structure, to methodology for making this structure, and to apparatus which implement the making methodology.
Rebar is commonly used to reinforce concrete structures such as roads, bridges, tunnels, airport runways, levies, and parking decks among others. In such structures, the rebar is embedded within the concrete and the external surface of the rebar is often ribbed in order to mechanically bind it to the concrete. The concrete provides compression strength (roughly speaking, resistance to compression) and the rebar provide tensile strength (resistance to pulling). Concrete is the most widely-used, man-made, construction material today.
Rebar products made out of composites of fibers and resin provide several advantages over those made out of steel. First and foremost, composite-material rebar does not experience the corrosion and degradation that steel does in some concrete environments. As the steel rebar corrodes it loses strength, thereby becoming less effective at carrying tensile loads. Further, as the rebar corrodes it expands substantially and “blows apart” the surrounding concrete mass, thereby rendering the concrete less effective at carrying compressive loads.
Such failing structures cannot be repaired from the inside and often have to be replaced entirely. This replacement is often costly, both financially as well as environmentally. The financial cost is apparent. The environmental cost becomes clear when one considers the environmental impact of manufacturing and transporting the replacement concrete and cement, which cannot be recycled. Therefore, while composite-material rebar may be nominally more expensive than steel rebar, using composite-material rebar will ultimately result in a less expensive overall structure due to the extended lifespan of the reinforced concrete that will not need to be replaced.
Composite-material rebar products that have a hollow center along their long axis confer several advantages over solid composite-material rebar products. The tensile strength of solid composite rebar does not increase linearly with the diameter of the rebar. Therefore, to increase the tensile strength considerably the rebar must be made thicker than one would otherwise expect. Increasing the diameter of the rebar increases the material cost of the product and may make the rebar more susceptible to failure due to load-carrying effects that differ between the fibers at the center and those at the edge of the rebar.
In contrast, hollow composite-material rebar will not increase in cost as much with an increase in diameter as a hollow product will be composed of much less material than a solid product. Further, by making the rebar hollow and omitting the fibers at the center of the rebar, there will be less discrepancy in load between the fibers of the rebar.
Another benefit to hollow composite-material rebar is the strength of the rebar can be adjusted by varying the thickness of the wall of the rebar without changing the outer diameter of the rebar. In contrast, increasing the strength of solid rebar by increasing the outer diameter of the rebar will result in correspondingly less concrete in a similarly sized structure, and therefore, less compression strength. Thus, the tensile strength of the hollow rebar may be adjusted without an effect on the compression strength of the surrounding concrete.
Finally, hollow rebar can provide a conduit or passageway through a reinforced concrete structure not available if solid rebar is used. Such a passageway could be used for the flow of different fluids, electrical cables, fiber optic cables, as well as for accommodating the internal employment of information yielding sensors, among others.
A natural question to ask when considering hollow rebar is, “Will it collapse?” It is possible that compressive radial forces may cause a hollow rebar element to collapse. Further, forces that pull on the ends of a hollow rebar element may cause the rebar to collapse radially, much as a Chinese finger puzzle. If a rebar element were to collapse radially, the external surface of the rebar would necessarily pull away from the corresponding internal surface of the concrete to which the rebar was bonded. This may cause a dramatic failure where the rebar is pulled out of the concrete structure.
The present disclosure provides a hollow composite-material rebar structure than may have enhanced radial strength and resistance to pull-out. The present disclosure further provides for an apparatus capable of producing such a hollow composite-material rebar structure and a method of using said apparatus.